Electromechanical oscillators



july 7, 1970 c. F. CLIFFORD 3, r H

ELECTRQMECHANICAL OSCILLATORS Filed Sent. 26, 1966 2 Sheets-Sheet l MIVE/(Tol? C t /z. F. Cummo MKWM/ Juiy 7, 1970 c. F. CLIFFORD 3,51

ELECTROMECHANICAL OSCILLATORS Filed Sept. 26, 1966 2 Sheets-Sheet 2 Mae/me Gee/1. F: CuFFa/b United States Patent 3,519,856 ELECTROMECHANICAL OSCILLATORS Cecil Frank Clifford, Newbridge Works, Bath, Somerset, England Filed Sept. 26, 1966, Ser. No. 582,036 Claims priority, application Great Britain, Oct. 15, 1965, 43,783; Apr. 18, 1966, 16,814 Int. Cl. H02r 7/06 US. Cl. 31022 2 Claims ABSTRACT OF THE DISCLOSURE A tuning fork oscillator in which the fork and the magnetic field are so placed that at least a part of the magnetic flux passes through at least parts of the fork tines, signal and drive coils for connection to a drive circuit are inductively linked to one or both of the tines, and the tines are provided with magnetic projections which cooperate with a wavy magnetic track or tracks on an escape wheel to drive the escape wheel as the fork oscillates.

This invention relates to electromechanical oscillators of the kind employing a tuning fork which is maintained in oscillation by electrical means.

Electromechanical oscillators in which a tuning fork forms the controlling element and determines the frequency of oscillation are in themselves known, and in the present state of development it is common practice to use a transistor as a single stage amplifier to maintain the work in oscillation. In one known type of oscillator a small permanent magnet is attached to each tine of the fork and two stationary coils are so placed that the fields of the magnets on the tines link with the turns of the respective coils. One of the coils is connected to the amplifier input so that the signals induced in the coil as the fork oscillates are amplified by the amplifier and the second coil is connected in the output circuit of the amplifier so that the output signals produce a series of magnetic impulses which drive the fork and maintain it in oscillation.

An inversion of this type of oscillator is known in which a coil is mounted on each tine of the fork and each coil cooperates with a fixed permanent magnet, the coils being. connected to the amplifier as previously described. In the first of the constructions described above it is necessary to keep comparatively heavy magnets in vibration. It is evident that the larger and more powerful the magnets the larger will be the signal induced in the signal coil and the greater will be the magnetic driving impulse imparted by a given electrical impulse in the driving coil. In the inversion described above large and powerful magnets may be used since they are fixed, but it is necessary to make electrical connections to vibrating coils and vibrating connections are not a desirable feature of any apparatus since they are liable in time to fracture, and they may in any case affect the oscillating frequency of the fork in a variable manner.

With either of the constructions referred to above there is a steady magnetic flux through each of the coils and this is only varied to a certain degree below and above a mean as the fork vibrates. It is therefore desirable to make the magnets as large as possible and provide coils with as many turns as possible to provide substantial signals for the amplifier and substantial driving impulses in the driving coil.

The object of the invention is to provide an improved construction of tuning fork oscillator in which only a single magnetic field is required, in which the means to provide the magnetic field are fixed, the coils are fixed and surround one or both tines of the fork, and at least 3,519,856 Patented July 7, 1970 'ice a part of each tine of the fork is also used as a part of the main magnetic circuit.

The invention thus provides a simple and technically advantageous construction of tuning fork oscillator. In one embodiment according to the invention a very large change of flux, and in fact a flux reversal, is produced as the tuning fork vibrates.

The invention consists of an electromechanical oscillator comprising a tuning fork, means to set up a magnetic field so positioned that at least a part of the magnetic flux passes through at least parts of the tines of the fork, a signal coil for connection in the input circuit of an amplifier, and a drive coil for connection in the output circuit of the amplifier, the signal and drive coils surrounding at least one of the tines of the fork, whereby in use mechanical oscillation of the tines of the fork induces signals in the signal coil which are amplified iind applied to the drive coil to maintain the fork in oscilation.

Preferably the means to set up the magnetic field comprises a permanent magnet.

In one embodiment an escape wheel is provided having a wavy magnetic track of known kind formed around its two faces, one tine of the tuning fork being extended and bent towards the other tine so that the ends of the tines are close to each other and the fork constitutes a gapped magnetic circuit, the escape wheel being mounted to rotate about an axis substantially parallel to the tines of the fork, the waves of the two magnetic tracks being phase-displaced by one half wave with respect to each other, whereby oscillation of the tines of the fork cause the escape wheel to rotate.

In another embodiment an escape wheel having a single wavy magnetic track formed around one face is mounted to rotate about an axis perpendicular to a centre line lying between the tines of the fork and parallel thereto, the ends of the tines being provided with magnetic projections which lie close to the wavy magnetic track, the Wavy magnetic tr-ack having an even number of waves.

In any of the embodiments in which an escape wheel is combined with the tuning fork the oscillator may be caused to provide an electrical tone signal by driving the escape wheel, so that the escape wheel will cause the tines to oscillate at their natural frequency.

Preferred embodiments of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 shows one embodiment including an escape wheel having wavy magnetic tracks around its two faces;

FIG. 2 shows an alternative embodiment using an escape wheel having a single wavy magnetic track formed around one face;

FIG. 3 shows an embodiment in which a large change of flux and a fiux reversal are provided; and

FIG. 4 shows an embodiment which is a modification of that of FIG. 3.

Referring to FIG. 1 of the drawings, a tuning fork, generally indicated by reference 11, is made of a single strip of a ferromagnetic material which is bent so as to form tWo tines, respectively 12 and 13, which lie parallel to each other. The tine 13 is of extended length as compared with the time 12 and its end portion 14 is bent at right angles. Pole pieces 15 and 16 are attached respectively to the end of the tine 12 and the end of the portion 14 forming part of the extended tine 13. Preferably the tuning fork is made of a material having a zero temperature coefiicient of elasticity, examples of such materials being nickel iron alloys known as Ni Span C and Ni Span D, and the pole pieces 15 and 16 are preferably made of a nickel iron magnetic alloy of high permeability, such as that known under the registered trademark Numetal.

The fork is carried by a support 22 which is attached to the centre of the curved portion between the tines, which forms the node of the tuning fork. The support 22 is so contrived that it is rigid against all deflecting forces except torsional forces, in response to which it may yield slightly. These torsional forces tend to rotate the fork support about an axis which is perpendicular to the upper face 23 in the drawing. As a result of this slight torsional yielding, the torsion member is incapable of transmitting to the mounting base vibrations which may arise due to out-of-balance tine forces. If the oscillator is arranged to operate at a frequency of 100 cycles per second then the transmitted vibrations would have a frequency of 100 cycles per second and harmonics thereof, and it is desirable that these should be suppressed.

A fixed signal coil 17 surrounds the tine 13 and a fixed drive coil 18 surrounds the tine 12, the coils being inductively linked to the respective tines.

Since the fork is made of a ferromagnetic material and one time is extended so as to approach closely the end of the other tine, the fork forms a complete magnetic circuit having a gap between the pole pieces and 16. It will, therefore, be evident that both the signal and drive coils could be placed on one tine and could, in fact, consist of a single winding with a tapping point. Where a tapped winding is employed the tapping is connected to the ground or earth line of the amplifier, one free end of the winding is connected in the input circuit and the other free end is connected in the output circuit of the amplifier. Naturally, the number of turns in the winding and the position of the tapping point are chosen to provide the desired impedances at the input and output of the amplifier.

A permanent magnet 19 is placed between the two tines 12 and 13 with its poles, respectively marked N and S, facing the respective tines.

An escape wheel comprises a disc 20, and is mounted to rotate about an axis 21 lying substantially parallel to the tines of the fork and the escape wheel is provided around each of its faces with a wavy magnetic track which is of a well-known type. The two tracks are concentric with the axis 21 and lie between the pole pieces 15 and 16.

The wavy magnetic tracks may be produced by coining the disc so that they are raised above its surfaces. Each track is formed by a number of raised teeth 24 and a number of depressions 25, so that a raised wavy track 26 is formed on each face.

Since the two tines of the fork always move in opposite directions during oscillation the two wavy tracks on the escape wheel must be phase-displaced with respect to each other by one half the pitch of one wave of the wavy tracks, one track facing each of the pole pieces 15 and 16, the two wavy tracks being, of course, magnetically connected together. They may be formed by making two separate discs each with one track and mounting them together, with the required phase-displacement.

An amplifier is provided, but this is not shown in the drawing. It may conveniently consist of a single transistor, powered by a single dry cell, and in that case the signal coil is connected in the input circuit of the amplifier while the drive coil is connected in the output circuit. The transistor may be connected in the common base mode, in which case the signal coil is connected in the emitter circuit and the drive coil is connected in the col lector circuit, or the amplifier may be connected in the common emitter mode, in which case the signal coil is connected in the base circuit and the drive coil is connected in the collector circuit.

In operation, when the amplifier is switched on a small initial current flows in the drive coil, which causes the tine 12 to make a small movement and the corresponding movement of the tine 13 induces a signal in the signal coil 17 which is amplified and applied to the drive coil 18 so that the fork begins to oscillate and is maintained in oscillation by the amplifier.

Since the two poles of the magnet 19 face the respective tines 12 and 13 a part of the flux of the magnet passes through the parts of the tines within the signal and drive coils, and a part of the flux passes through the other parts of the tines and across the gap between the pole pieces 15 and 16. As the fork oscillates the gap between each tine and the adjacent pole of the magnet is cyclically varied, and the fiux in the fork is also varied. The wavy magnetic track of the escape wheel 20 causes the escape wheel to rotate at a fixed speed, one undulation of,the wavy magnetic track passing between the pole pieces 15 and 16 for each complete oscillation cycle of the tuning fork.

FIG. 2 shows an embodiment in which an escape wheel having a single wavy track formed around one face is employed. A tuning fork, generally indicated by refererice 27, supported on a supporting member 28, is similar to the tuning fork 11 except for the omission of the extended portion 14 and the fact tha the ends of the tines are cut oif squarely.

The tuning fork 27 is provided with two coils, respectively 29 and 30, as in the embodiment of FIG. 1, and these coils may be precisely similar to the coils 17 and 18 of FIG. 1. A permanent magnet 31 lies between the tines 32 and 33 of the fork and serves the same purpose as the magnet 19 of FIG. 1. An escape wheel, indicated generally by reference 34, has a single wavy magnetic track formed on its upper face 35. The escape wheel 34 is'mounted to rotate about an axis 36, which is perpedicular to a centre line lying between the tines 32 and 33 and parallel thereto. The tine 32 has attached to it a magnetic pin or extension 37, while the tine 33 has attached to it a magnetic pin 38. These two pins each project beyond one edge of the respective tine and lie close to the face of the wavy magnetic track. They may be made of permanent magnet material if desired. Hence, if the coils 29 and 30 are connected in circuit with an amplifier, as previously described, the tines will be caused to oscillate and will thereby drive the escape wheel 34. On the other hand, if the escape wheel 34 is driven it will cause the tines 32 and 33 to oscillate so that signal impulses are induced in one of the windings contained in the coils 29 and 30, the speed of rotation of the escape wheel 34 will be controlled by the natural frequency of the tuning fork, and an amplified signal corresponding to the fork frequency will be available at the amplifier output.

It will be appreciated that various modifications may be made in the two embodiments described above. For example, if it is not required to provide a drive for an escape wheel the tine 13 need not be extended and the pole pieces 15 and 16 may be omitted.

In FIG. 3 there is shown the operative end of an electromechanical oscillator including a tuning fork, comprising a large magnet 38 of parallelopipedic form, which is partly shown in full lines and partly shown in dotted lines 384. This magnet is magnetized between two of its faces as indicated by the N and S markings 39*. A yoke, generally indicated by reference 40, is fixed in contact with the N pole face and comprises a body portion 41 with projecting portions 42 and 43, parts of which are turned down to form pole pieces, respectively 44 and 45. At the other end of the body portion 41 are two further extensions respectively 46 and 47, the ends of which are turned down to form two further pole pieces respectively 48 and 49. The extensions 46 and 47 are longer than the extensions 42 and 43, so that the pole pieces 44 and 45 are closer to the magnet 38 than the pole pieces 48 and 49. An exactly similar yoke, generally indicated by reference 50, is attached to the S pole face of the magnet but is placed in inverted relationship, and the other way round, so that the pole pieces formed on the short extensions, of which only one, reference 51, is seen, lie inside the pole pieces 48 and 49 of the yoke 40. Similarly, the pole pieces 52 and 53 formed at the ends of the long extensions of the yoke 50' lie outside the pole pieces 44 and 45 of the yoke 40. In this way four pairs of pole pieces are provided, there being a gap between the pole pieces of each pair. The magnetic polarity (N or S) is marked on each of the seven pole pieces which are visible in the drawing.

Only the ends of the tines of the tuning fork are shown, these being respectively 54 and 55, and each of the tines lies within two of the gaps formed by the pole pieces described earlier. A fixed coil 56 surrounds the tine 54 and a fixed coil 57 surrounds the tine 55.

When the fork is at rest the tine 55 is in a neutral position in the centre of the gap between the pole pieces 48 and 51 and also in the centre of the gap between the pole pieces 44 and 52. Flux from the magnet 38 passes across the gap between the pole pieces 48 and 51 and flux from the magnet also passes between the pole pieces 44 and 52. Flux also tends to pass from the pole piece 48 along the tine to the pole piece 52 and from the pole piece 44 along the tine in the other direction to the pole piece 51, but it will be evident that these two fluxes will oppose and cancel each other. Similar conditions obtain in respect of the other tine.

When the fork is vibrating the tine 55 first moves inwardly and approaches the pole pieces 44 and 51 and recedes from the pole pieces 48 and 52. The alteration in the lengths of the air gaps is such that an increasing flux passes from the pole pieces 44 along the tine 55 and to the pole piece 51. This flux reaches a maximum when the tine 55 has moved to its maximum extent in this direction. The flux from the pole piece 44 through the tine to the pole piece 51 diminishes as the tine moves in the other direction until, as the tine passes through the neutral position, it has fallen to zero, and as the tine moves closer to the pole pieces 48 and 52 flux begins to pass from the pole piece 48 along the tine to the pole piece 55. This flux is in the opposite direction through the tine. This flux also reaches a maximum at the point at which the velocity of the tine falls to zero. It will thus be evident that, in place of the small change of flux which takes place in the conventional tuning fork arrangement, a large change of flux, including a flux reversal, takes place in the oscillator according to this embodiment of the invention. Consequently a very much larger signal is induced in the coil 57 and if this is in the input circuit of the amplifier it provides a much larger input. Exactly the same effects are produced by the movement of the other tine 54.

It will be evident that in view of the large change in flux which takes place as the fork vibrates a correspondingly smaller output signal from the amplifier through the coil 56 will be sufiicient to provide the necessary mechanical driving power to maintain the fork in oscillation.

This results in greatly increased sensitivity in the oscillator and correspondingly reduced power consumption, a matter of the greatest importance in very tiny units intended, for example, for incorporation in wrist watches.

FIG. 4 shows an embodiment which is a modification of that shown in FIG. 3. It comprises a magnet 38 similar to that of FIG. 3 having north and south pole faces as indicated by the markings 39. A yoke 58 has a body portion 59 with projecting portions 60 and 61 at one end having end portions 62 and 63 which are turned down. At its other end the yoke 58 has projecting portions 64 and 65 having end portions 66 and 67 which are turned down. A second yoke 68 is exactly similar to the yoke 58 but is placed in inverted relationship, and the other way round, so that the short extensions of each yoke are adjacent the long extensions of the other, the visible turned ends of the yoke 68 being indicated by references 69,

6 70 and 71. The two tines of the tuning fork are indicated at 72 and 73 and the two coils are shown diagrammatically at 74 and 75.

The construction of FIG. 4 is similar to that of FIG. 3 except that the turned down ends of the extensions are shorter and the tines of the fork are narrower. It functions in exactly the same way as that of FIG. 3 and provides a flux reversal.

I claim:

1. An electromechanical oscillator comprising a tuning fork having straight tines of equal length, a single permanent magnet lying between the tines of the fork adjacent their ends, signal and drive coils surrounding at least one tine of the fork, an escape wheel mounted to rotate about an axis perpendicular to a centre line lying between the tines of the fork and parallel thereto, the escape wheel having around one face thereof a continuous wavy magnetic track with an even number of waves, and projections on the ends of the tines of the fork made of a magnetic material of high permeability and low retentivity so placed that their ends are close to diametrically opposite points on the wavy magnetic track, whereby, during oscillation of the tines of the fork, a part of the magnetic flux of the magnet passes through the tines of the fork to provide inductive coupling with the coils for maintaining oscillation of the tines and the other part of the magnetic flux passes through the wavy magnetic track to cause rotation of the escape wheel.

2. An electromechanical oscillator comprising a tuning fork which is maintained in mechanical oscillation at its natural frequency by electrical means, means to set up a single magnetic field so positioned that a part thereof passes through each tine of the fork, one tine of the fork being extended and bent round so that the ends of the tines face each other, projections on the ends of the tines made of a material of high magnetic permeability and low retentivity of such length as to leave a gap therebetween, signal and drive coils for electrically maintaining the mechanical oscillation of the fork surrounding at least one of the fork tines and being thereby linked with the said single magnetic field, an escape wheel provided with wavy magnetic tracks on each of its faces so mounted that the wavy magnetic tracks pass through the gap and the wheel rotates about an axis lying substantially parallel to the tines of the fork, the wavy magnetic tracks being magnetically connected and rotationally displaced one from the other by one half the pitch of one wave, whereby a part of the said single magnetic field passes through a part of the wavy magnetic tracks so that oscillation of the fork tines causes rotation of the escape wheel and the said single magnetic field serves both for maintaining the fork in oscillation and for providing a magnetic drive to the escape wheel.

References Cited UNITED STATES PATENTS 2,662,205 12/1953 Virkus et al. 318-330 3,277,644 10/1966 Nomura et a1. 58-23 3,208,287 9/1965 Ishikawa et al 74-1.5 3,322,016 5/1967 Ishikawa et a1. 58-23 3,212,252 10/ 1965 Nakai 58-23 2,928,308 3/1960 Godbey 84-409 XR 2,152,955 4/1939 Coyne 310-25 XR MILTON O. HIRSHFIELD, Primary Examiner B. A. REYNOLDS, Assistant Examiner US. Cl. X.R. 

